1) Field of the Invention
The present invention relates to a rigid, optically transparent, high index, impact resistant polythiourethane/urea material, which is particularly suited for making optical articles such as sun lenses, ophthalmic lenses and protective lenses.
2) Background of the Invention
Plastic materials are widely used in the optical field and particularly in the ophthalmic field for their lightness, high impact resistance and tintable capability by immersion in a bath containing an organic dye.
Optically transparent plastic materials having a high refractive index, higher than 1.53, are of major interest since they render it possible to manufacture optical articles such as lenses of lower thickness for an equivalent corrective power (optical power).
Of course, this increase in refractive index of the material shall not be at the expense of the other valuable properties such as transparency and impact resistance of the material.
Preferably, other required properties for the lens material are                non yellowness;        ability to be treated (by hard coats, primers, . . . );        density as low as possible—ageing resistance (especially photodegradation resistance).        
U.S. Pat. No. 6,127,505 discloses a transparent, non-elastomeric, high index, high impact resistant polyurethane material which is a reaction product of:                a polyurethane prepolymer prepared by reaction of an aliphatic or cycloaliphatic diisocyanate with at least one OH containing intermediate having a rate average molecular weight of from about 400 to 2.000 selected from the group consisting of polyester glycols, polycaprolactone glycols, polyether glycols, polycarbonate glycols and mixtures thereof, in an equivalent ratio of about 2.5 to 4.0 NCO/1.0 OH and        at least one first aromatic diamine curing agent selected from the group consisting of 2,4-diamino-3,5, diethyl-toluene, 2,6-diamino-3,5,diethyl-toluene and mixtures thereof in an equivalent ratio of about 0.85 to 1.02 NH2/1.0 NCO.        
Unfortunately, the polyurethanes obtained have relatively low refractive index,
      n    D    25    ,of at most 1.53.